Threshold of pain

Traditionally the threshold has often been defined, as we defined it formerly, as the least stimulus intensity at which a subject perceives pain. Properly defined, the threshold is really the experience of the patient, whereas the intensity measured is an external event. It has been common usage for most pain research workers to define the threshold in terms of the stimulus, and that should be avoided ... The stimulus is not pain (q.v.) and cannot be a measure of pain.

Although the phrasing may not convey it perfectly, the distinction clearly meant is the aforementioned one between the stimulus and the perception of it. The intensity at which a stimulus (e.g., heat, pressure) begins to evoke pain is thus called by a separate term, threshold intensity. So, if a hotplate on a person's skin begins to hurt at 42 °C (107 °F), that is the pain threshold temperature for that bit of skin at that time. It is not the pain threshold (which is internal/subjective) but the temperature at which the pain threshold was crossed (which is external/objective).

The intensity at which a stimulus begins to evoke pain varies from individual to individual and for a given individual over time.

Heat

The temperature at which heat becomes painful for a recipient is called the heat pain threshold for that person at that time. One study showed that morning-oriented people have higher pain threshold for heat as compared to evening-oriented individuals.